Nowadays, consumers are demanding more natural foods, obliging the industry to include natural antioxidants in foods. Natural antioxidants have been used instead of synthetic antioxidants to retard lipid oxidation in foods to improve their quality and nutritional value. This review discusses some aspects of recent research on antioxidant activity of plant extracts and natural compounds to improve meat quality. Many herbs, spices, and their extracts have been reported as having high antioxidant capacity, such as some plants of the Lamiaceae family, e.g., oregano (Origanum vulgare L.), rosemary (Rosmarinus officinalis L.), and sage (Salvia officinalis L.). The antioxidant activity of these plants is attributed to their phenolic compound content, which includes volatile compounds also known as essential oils. Several factors that cause some differences on the antioxidant activity of plant extracts include: type of solvent used during extraction, measurement method, and number of samples. Some studies have demonstrated that shelf-life and meat quality can be improved by using natural antioxidants in some stages of meat production. The main effects of these compounds are reducing microbial growth and lipid oxidation during storage. Nevertheless, more research is needed to determine antimicrobial activity of natural antioxidants in meat during storage, identify the main metabolic pathway of these compounds, and its effect on other meat quality parameters.

The increasing preference for natural foods has obliged the food industry to include natural antioxidants in various products to delay oxidative degradation of lipids, improve quality and nutritional value of foods, and replace synthetic antioxidants (Fasseas et al., 2007; Wojdylo et al., 2007; Camo et al., 2008). Including antioxidants in the diet has beneficial effects on human health because they protect the biologically important cellular components, such as DNA, proteins, and membrane lipids, from reactive oxygen species (ROS) attacks (Su et al., 2007). Synthetic antioxidants have been used to retard or minimize oxidative deterioration of foods, such as butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), and tertiary butyl hydroquinone (TBHQ) (Fasseas et al., 2007). Recently, consumers have rejected synthetic antioxidants because of their carcinogenicity (Altmann et al., 1986; Van Esch, 1986). The advantages of natural antioxidants in foods are high consumer acceptance and their safe use. The disadvantages are their higher cost and lower effectiveness (Fasseas et al., 2007).

The short shelf-life of refrigerated packed meat makes its commercialization more difficult. Some studies have demonstrated that meat shelf-life and quality can be improved by natural antioxidants added in the pre-slaughter and post-slaughter stages. That is, incorporating natural antioxidants in animal diets, onto the meat surface, or active packaging.

The aim of this review was to discuss some aspects of recent research on antioxidant activity of plant extracts and natural compounds and their use to improve meat quality.

DISCUSSION

Definition and chemistry of natural antioxidants Natural antioxidants are various substances with different chemical characteristics, which are widely present in plants. Antioxidants retard or inhibit oxidation of other substances by inhibiting the initiation or propagation of oxidizing chain reactions (Velioglu et al., 1998). Consequently, natural antioxidants can protect the biologically important cellular components from oxidative processes caused by reactive oxygen species (ROS) (Su et al., 2007).

The total antioxidant capacity of fruit and vegetable extracts reflects concentrations of ascorbic acid (vitamin C), alpha-tocopherol (vitamin E), beta-carotene (vitamin A precursor), various flavonoids, and other phenolic compounds (Pennington and Fisher, 2009).

The antioxidant capacity of vitamins and phenolic compounds measured by different methods indicates that some natural antioxidants have a high antioxidant activity, such as gallic acid, cyanidin, quercetin, catechin, thymol, and carvacrol (Table 1). Measurements are generally based on Trolox equivalents, which are a water-soluble derivative of vitamin E used as a standard.

Many herbs, spices, and their extracts are known to possess antioxidant effects (Zheng and Wang, 2001; Shan et al., 2005), especially clove (Myrtaceae), cinnamon (Cinnamomum zeylanicum Blume in Lauraceae), and oregano (Lamiaceae) (Shan et al., 2005). Some authors have attributed the antioxidant activity of these plants to their phenolic compound content, resulting in a positive linear correlation between them (Velioglu et al., 1998; Kähkönen et al., 1999; Zheng and Wang, 2001; Shan et al., 2005). Antioxidant activity, measured by three different methods, and total phenolic content of some plant extracts are shown in Table 3. Species with a strong antioxidant capacity have high phenolic content. One example is clove that had the highest antioxidant capacity as measured by three methods and was related to the highest phenolic content. Zheng and Wang (2001) found the strongest antioxidant activity in Mexican oregano (92.18 mmol of Trolox equivalents (TE) g-1 fresh weight) and the highest phenolic content (17.51 mg gallic acid equivalents (GAE) g-1 fresh weight). However, some studies did not obtain the same relationship between antioxidant capacity and total phenolic content in some species (Kähkönen et al., 1999; Wojdylo et al., 2007). For example, high antioxidant activity of apple (Malus domestica Borkh.) extracts with low total phenolic content may be due to the presence of other compounds that might interfere with the determination methods (Kähkönen et al., 1999). In addition to phenolic compounds, some species have shown a positive relationship between antioxidant activity and vitamin C, vitamin E, and beta-carotene content. Polyphenols and vitamin C are the major contributors of the high antioxidant activity of kiwifruits (Actinidia sp.) (Du et al., 2009). The same was detected in Brassica vegetables with carotenoids and Vitamin E responsible for up to 20% of total antioxidant activity (Podsędek, 2007).

Table 3. Antioxidant capacity and total phenolic compound content of some methanolic plant extracts and their effects on meat quality during 7-d refrigeration.

Differences found in some research indicate that other factors can affect the antioxidant activity of some plants (Velioglu et al., 1998), which can be related to number of samples, small differences between the highest and lowest values, method used to extract antioxidant compounds, and method used to measure antioxidant capacity (Kähkönen et al., 1999; Kulisic et al., 2004; Shan et al., 2005; Fasseas et al., 2007; Su et al., 2007; Wojdylo et al., 2007; Karadag et al., 2009; Tabart et al., 2009).

Some authors have reported that natural antioxidants have no effect on sensory characteristics of meat. Chaves et al. (2008) did not detect any effect of essential oil compounds (carvacrol and cinnamaldehyde), added to the diet of growing lambs, on the sensory characteristics of sirloins. The same was observed in pork where different essential oils were included in the diet of pigs (Janz et al., 2007). The only evidence of the effect of natural antioxidants on inhibiting off-odor formation and discoloration of meat is active packaging (Djenane et al., 2003; Nerín et al., 2006; Camo et al., 2008).

Some studies have used vitamin E (a-tocopherol) in both direct application onto meat and animal diet supplementation to extend lipid stability of fresh beef during storage (Arnold et al., 1993; Eikelenboom et al., 2000; OGrady et al., 2000). Mitsumoto et al. (1993) reported that the post-mortem addition of vitamin E in beef was less effective in retarding lipid oxidation than dietary supplementation. Djenane et al. (2002) concluded that surface application of vitamin C, taurine, rosemary, vitamin E, and combinations of the last three with vitamin C have a positive effect on oxidative stability of beef steaks packaged in a modified atmosphere.

Meat color has been reported as the most important factor when consumers assess meat quality since they relate color to freshness. However, color does not correspond to differences in eating satisfaction (Carpenter et al., 2001). Changes in meat color are due to oxidation of red oxymyoglobin to metmyoglobin (MMG), which give meat an unattractive brown color (Nerín et al., 2006). Some reports demonstrate that natural antioxidants can retard meat color loss by extending the red color (a*) and delaying MMG formation (Table 3). One example of dietary natural antioxidants affecting meat color is the higher color parameters (a* and yellowness b*) of meat from lambs fed with oregano essential oil supplementation (1 mL oregano essential oil kg-1) (Simitzis et al., 2008). Another example is the reduction of MMG formation and intense red color obtained in fresh beef steaks whose surface was sprayed with a rosemary and ascorbic acid solution during refrigeration (Djenane et al., 2003). Carpenter et al. (2007) noted that the color parameters (lightness L*, b*, and a*) of raw pork patties did not vary by adding grape seed and bearberry extracts. The same results were obtained for fresh chicken breast meat (Chouliara et al., 2007).

Some authors have demonstrated the antioxidant effect of some vitamins to delay color loss in meat during storage. One of these vitamins, vitamin E (a-tocopherol), has been used to improve myoglobin stability of fresh beef during storage by supplementing animal diets with at least 1300 IU d-1 (Arnold et al., 1993) or 2025 mg of vitamin E per day (Eikelenboom et al., 2000). A vitamin C solution of sodium ascorbate (5% of cut weight containing at least 4% of sodium ascorbate) injected in beef was also effective in improving color stability and extending the meats retail display life (Wheeler et al., 1996).

The antimicrobial properties of essential oils extracted from plants used against a wide range of microorganisms (Dean and Ritchie, 1987; Sivropoulou et al., 1996; Chao et al., 2000) have been considered as an alternative to antibiotics in livestock. Therefore, essential oils are promising as feed additives to improve feed efficiency and control the spread of pathogens in livestock (Benchaar et al., 2008).

Nowadays, active packaging has been intensely developed due to recent contamination outbreaks associated with meat products (Coma, 2008). Table 3 shows some effects of natural antioxidants included in active films. The effectiveness of antimicrobial compounds might be higher when they are incorporated in an active film applied to the food surface than if directly applied to the food surface by spraying or dipping (Coma, 2008; Zinoviadou et al., 2009). Oregano-based films have been effective against bacterial growth in meat. For example, active films containing oregano essential oil can reduce the growth of total flora and pseudomonas, thus inhibiting the growth of lactic acid bacteria in beef (Zinoviadou et al., 2009). Moreover, oregano-based films were effective against Salmonellatyphimurium and E. coli O157:H7 inoculated in beef muscle slices (Oussalah et al., 2006). Thus, beef shelf-life can be increased by 2 when employing active films containing 1.5% oregano oil (w/w) (Zinoviadou et al., 2009). Skandamis and Nychas (2001) informed that oregano essential oil compounds affected both growth and metabolic activity (glucose and lactate consumption) of meat microorganisms during storage. In addition, meat packed in active film containing natural antioxidants exhibits higher antioxidant activity than meat packed in films without antioxidants (Nerín et al., 2006; Camo et al., 2008). Oregano-based films stabilized lipid oxidation in beef muscle slices during 7 d when stored at 4 °C (Oussalah et al., 2004). There are few studies about the effect of active film containing natural antioxidants on meat color. Camo et al. (2008) reported a greater effect of oregano active film against color loss and MMG formation in lamb meat than rosemary active packaging. The latter was less effective than directly adding rosemary extract onto the meat surface. Values of a* were above 10 and MMG formation was around 40% after 13 d of refrigeration using oregano active films and directly adding rosemary extract onto the meat surface. Zinoviadou et al. (2009) reported smaller changes in total color difference (DE) and saturation difference (Dchroma) in beef cuts wrapped in films with different levels of oregano oil (0.5%, 1.0%, and 1.5% w/w) during refrigeration and resulted in better color retention due to oxymyoglobin stabilization .

CONCLUSIONS

Some plant extracts are an excellent source of natural antioxidants that can improve meat shelf-life and quality mainly by retarding lipid oxidation and microbial growth. The effect of oregano essential oil on meat quality has been studied the most, whereas there is less information about other plants.

Antimicrobial activity of spices and herbs has been extensively studied. However, most of the research was done in in vitro assays using microbiological media rather than actual foods. Further research is needed to determine their effect on bacterial growth in raw and cooked meats during storage. Active packaging containing natural antioxidants is a promising tool in this field.

There is some evidence that dietary essential oils can improve meat quality. Since bioavailability of essential oils in meat cannot be directly demonstrated, more research is needed to identify the main metabolic pathway of these compounds and the key essential oil antioxidant compounds deposited in meat.

Further research is needed to determine the effect of natural antioxidants on other meat quality characteristics.